Over the years, bodily characteristics have been determined by obtaining a sample of bodily fluid. For example, diabetics often test for blood glucose levels with a blood glucose meter. Traditional blood glucose determinations have utilized a painful finger prick using a lancet to withdraw a small blood sample that is used by the blood glucose meter. This results in discomfort from the lancet as it contacts nerves in the subcutaneous tissue. The pain of lancing and the cumulative discomfort from multiple needle pricks is a strong reason why patients fail to comply with a medical testing regimen used to determine a change in characteristic over a period of time. In addition, these blood glucose meters are only designed to provide data at discrete points and do not provide continuous data to show the variations in the characteristic between testing times.
A variety of subcutaneous electrochemical sensors for use with monitors have been developed for detecting and/or quantifying specific agents or compositions in a patient's blood. For instance, glucose sensors have been developed for use in obtaining an indication of blood glucose levels in a diabetic patient. Such readings are useful in monitoring and/or adjusting a treatment regimen which typically includes the regular administration of insulin to the patient. Thus, blood glucose readings from the monitor improve medical therapies with semi-automated medication infusion pumps of the external type, as generally described in U.S. Pat. Nos. 4,562,751; 4,678,408; and 4,685,903, which are herein incorporated by reference. Typical thin film sensors are described in commonly assigned U.S. Pat. Nos. 5,390,671; 5,391,250; 5,482,473; and 5,586,553, which are incorporated by reference herein. However, the thin film subcutaneous glucose sensor must be changed every few days to prevent infection. Also, due to the small size of this sensor to minimize pain on insertion under the skin, the enzymes on the sensor wear out relatively quickly and require regular replacement. In addition, the user must carry around external hardware connected or linked to the sensor. Thus, although subcutaneous sensors provide an improvement over conventional test strips, they still require frequent changes.
Long term implanted glucose sensors have been proposed that can stay in the body for long periods of time, such as weeks and months. These long term implanted glucose sensors are particularly well adapted for use with automated implantable medication infusion pumps, as generally described in U.S. Pat. No. 4,573,994, which is herein incorporated by reference. The long term glucose sensor would obviate the need for frequent replacement of sensors and the need to carry around a large amount of external equipment. However, the long term glucose sensor is typically placed in the superior vena cava of the patient's body, and the insertion and placement of the long term sensor in this location is quite invasive to the body and requires much effort by an attending physician. As a result, removal and replacement of long term sensors in the superior vena cava is difficult.
Long term glucose sensors may be adapted for insertion and placement in locations that are less invasive to the body and require less effort by the attending physician, such as the peritoneal, subcutaneous, and/or adipose tissue of the patient's body. For example, an outer covering may be formed around the long term sensor, and the long term sensor formed with the outer covering may then be placed into the peritoneal, subcutaneous, or adipose tissue of the patient's body. After insertion, the long term sensor would not be usable for a period of time until the body heals and vascularizes the implanted long term sensor. Thus, each time a long term sensor formed with an outer covering is replaced, the body must re-vascularize the replaced sensor. Another drawback to long term sensors is the development of scar tissue at the insertion site that encapsulates the implanted sensor formed with the outer covering. Therefore, materials must be carefully selected to promote vascularization and not encapsulation. This requires careful construction of the outer covering for the long term sensor, which increases costs and may further delay the period of time before a newly implanted sensor may be used. Further, extensive surgery may be required to cut through the scar tissue and remove the implanted sensor, thus rendering the insertion site unusable for implantation of a replacement sensor. Accordingly, to minimize and/or eliminate the delay due to re-vascularization and the need for extensive surgery due to encapsulation, it would be desirable to develop a reusable sensor insertion site for use with a replaceable sensor.